Electrowetting module

ABSTRACT

An electrowetting module ( 20 ) comprises a fluid chamber ( 8 ) which contains a first fluid (A) and a second fluid (B), which are separated by an interface ( 14 ), and means to ( 16,17 ) exert a force on at least one of the fluids to change the position and/or shape of the interface. By providing at least one of the fluids with a compound having at least one aromatic, non-fused, residue, the performance of the module can be enhanced. For example the optical power of an electrowetting lens ( 30 ) can be increased.

The invention relates to an electrowetting module, comprising a fluidchamber, which contains at least a first body of a first fluid and asecond body of a second fluid, said bodies being separated by aninterface, and means for exerting a force on at least one of the bodiesto change the position and/or shape of the interface.

It is observed that wetting techniques make it possible to manipulate avolume of a fluid along a predetermined path. With these techniques, thesurface tension of said volume is locally altered (usually reduces),causing the volume to flow in the direction of its lowest surfacetension.

Further, it is observed that a fluid is a substance that alters itsshape in response to any force, and includes gases, vapors, liquids andmixtures of solids and liquids, capable of flow.

The term “wettability” of a surface by a certain fluid gives anindication of the ease with which said fluid may wet said specificsurface, which may for instance depend on the nature of and/or theelectric potential across said surface. If a surface has a “highwettability” by a specific fluid, this indicates that a droplet of saidfluid in contact with said surface will have a rather expanded shape,with a relatively large contact area and a relatively small contactangle, usually less than about 90°. “Low wettability” indicates that thedroplet in contact with said surface will have a rather contractedshape, with a relatively small contact area and a relatively largecontact angle, usually exceeding about 90°.

The term “wetting” is understood to encompass all techniques causing thesurface tension of a volume, e.g. a droplet of a specific fluid to belocally varied, so as to influence the wetting behavior of said fluidwith respect to a specific surface.

In modules wherein use is made of the wettability phenomenon, it isnecessary that the two fluids have desired properties, for example:densities as close as possible; low melting points; adapted viscosity;good electrowetting behavior; non poisonous; and, in case of an opticalmodule, indices of refraction of a certain predetermined difference.

An example of such an optical module is an electrowetting-based lens,also called an electrowetting lens, of which the focal distance can bechanged. In an electrowetting lens the interface between the two fluidbodies is a meniscus. In such a module the first fluid body is anelectrically conducting and/or polar liquid and the second fluid body isan electrically non-conducting liquid. The first liquid is, for examplesalted water and the second liquid is, for example an organic non-polar,water-inmiscible liquid such as bromo-decane, chloro-or bromonaphtaleneand silicone oil. The electrowetting optical module is provided withmeans for exerting an electrical force by means of which the shape andor the position of the meniscus can be shaped. Other examples of theelectrowetting optical module are zoom lenses, diaphragms, diffractiongratings, filters and beam deflectors. Embodiments of these modules aredescribed in PCT patent application no. IB03/00222 and in Europeanpatent applications nos. 020789309.2, 02080387.0 and 02080060.3. Theelectrowetting optical modules are very compact and may therefore beused with much advantage in devices, like optical disc scanning devices,mini cameras for a/o mobile phones, displays etc.

The optical power of an optical electrowetting module is determined bythe curvature of the meniscus and the difference between the refractiveindex of the first liquid and that the second liquid. There is a growingdemand for optical electrowetting modules, which can produce largeoptical power variations. Since the maximum change in curvature of themeniscus is determined by the size of the electrowetting cell, thechange in optical power that can be realized by change the curvature islimited for a given electrowetting lens. The problem of increased powerthus should be solved in another way.

Another electrowetting module is a motor which uses the electrowettingeffect to manipulate a volume of fluid along a predetermined path, whichfluid causes two motor elements to move relative to each other, as willbe described later on. In such a motor one of the fluids may beflattened due to centrifugal forces, if the densities of the first fluidand of the second fluid are not matched to each other.

It is an object of the invention to provide an electrowetting module asdefined in the opening paragraph, which module, if used as an opticalmodule, allows varying the optical power over a larger range and, ifused in a motor, is considerably less sensitive to flattening of one ofthe fluids. The electrowetting module is characterized in that at leastone of the fluids comprises a compound having at least one aromatic,non-fused, residue.

This electrowetting module is based on the insight that such a compound,when used as or when included in the non-conductive, or non-polar, fluidor liquid, increases the refractive index of this fluid or liquid sothat the difference between the refractive indices of the non-conductiveand of the conductive fluids in an electrowetting optical module isincreased. In this way the optical power and the range of powervariation can be increased. If the said compound is used as or includedin the non-polar fluid in an electrowetting motor, it preventsflattening of the fluid.

Another aspect of the invention is that for an optical module thecurvature of the meniscus can be decreased while maintaining the opticalpower. In this way the sensitivity for optical aberrations of the modulecan be reduced. Moreover the actuating voltage needed for a requiredchange in the optical power can be reduced.

It is remarked that an electrowetting lens with fluid bodies showing anincreased refractive index difference is disclosed, for example by B.Berge and J. Peseux in Eur. Phys. J. E3, 159-163 (2000). The fluidbodies of this lens consist of water and chloronaphtalene respectively.These lens, however do not show good electrowetting behavior, especiallynot for DC voltages.

A group of compounds has been traced, which provide fluid or liquidswith refractive indices and/or densities which are larger than knownfluids and thus are very suitable to be used as or to be included in atleast one of the fluids of the electrowetting module of the invention.Preferred compounds are defined in claims 2 to 6.

A module comprising such a compound may be configured as an opticalcomponent, the first and said second fluid body having differentrefractive indices. In such an optical module the compound added to oneof the fluids has an refractive index difference increasing effect.

In such a module the first fluid body may be electrically conductingand/or polar, and the second fluid body may be electricallynon-conducting and the module may be provided with means for exerting anelectric force to change the position and/or shape of themeniscus-shaped interface.

The difference in refractive index is from 0,1 to 0,3, preferably from0,1 to 0,2; the refractive index of said second, non-conducting bodybeing larger than 1.4, preferably greater than 1.5.

Preferably the first and second fluid bodies show a substantiallysimilar density.

The module may also be provided with means for exerting a pressure tochange the position of the interface

These and other aspects of the invention will be apparent from andelucidated by way of non-limitative example with reference to theembodiments described hereinafter and illustrated in the accompanyingdrawings.

In the drawings:

FIG. 1 shows, in a cross-section through its optical axis, a knownelectrowetting lens in a non-activated state;

FIG. 2 shows such a lens in an activated state;

FIG. 3 shows a lens according to the invention in an activated state,and

FIGS. 4 a and 4 b shows, in a cross-sectional view, an activatedelectrowetting motor at two different time moments.

FIG. 1 shows an electrowetting module constituting a variable focuslens. The element comprises a first cylindrical electrode 2 forming acapillary tube, sealed by means of a transparent front element 4 and atransparent rear element 6 to form a fluid chamber 8 containing twofluids. The electrode 2 may be a conducting coating applied on the innerwalls of a tube.

In this embodiment of the electrowetting module the two fluids consistof two non-miscible liquids in the form of an electrically insulatingfirst liquid A, currently, for example a silicone oil or an alkene, andan electrically conducting second liquid B, currently, for example,water containing a salt solution. The first fluid A has a higherrefractive index than the second fluid B.

The first electrode 2 is a cylinder of inner radius typically between 1mm and 20 mm. This electrode is formed of a metallic material and iscoated by an insulating layer 10, formed for example of parylene. Theinsulating layer has a thickness of between 50 nm and 100 μm. Theinsulating layer is coated with a fluid contact layer 12, which reducesthe hysteresis in the contact angle of the meniscus 14, i.e. theinterface between the fluids A and B, with the cylindrical wall of thefluid chamber. The fluid contact layer is preferably formed from namorphous fluorocarbon such as Teflon™ AF1600 produced by DuPont™. Thefluid contact layer 12 has a thickness between 5 nm and 50 μm.

A second, annular, electrode 16 is arranged at one side of the fluidchamber, in this case, adjacent the rear element 6. The second electrodeis arranged with at least one part in the fluid chamber such that theelectrode acts on the second fluid B.

The two fluids A and B are non-miscible so as to tend to separate intotwo fluid bodies separated by a meniscus 14. When no voltage is appliedbetween the first and second electrodes, the fluid contact layer 12 hasa higher wettability with respect to the first fluid A than with respectto the second fluid B. FIG. 1 shows this lens configuration, i.e. thenon-activated state of the electrowetting lens. In this configuration,the initial contact angle θ between the meniscus and the fluid contactlayer 12, measured in the fluid B, is larger than 90°. Since therefractive index of the first fluid A is larger than the refractiveindex of the second fluid B, the lens formed by the meniscus, herecalled meniscus lens, has a negative power in this configuration.

Due to electrowetting, the wettability by the second fluid B variesunder the application of a voltage between the first electrode and thesecond electrode, which tends to change the contact angle. FIG. 2 showsthe lens configuration if such a voltage from a source 17 is supplied tothe lens, i.e. if the lens is in the activated state. In this case thevoltage is relatively high, for example between 150V and 250V. and themeniscus has now a convex shape. The maximum contact angle θ between themeniscus and the fluid contact layer 12 is, for example of the order of60°. Since the refractive index of fluid A is larger than fluid B, themeniscus lens I in this configuration has a positive power and itfocuses an incident beam b in a focal spot 18 at a certain distance dfrom the lens.

For further details about the construction of the variable focus lensreference is made to international patent application no. IB03/00222. Azoom lens, which comprises at least two independently controllableinterfaces between a higher refractive index liquid and lower refractiveindex fluid, is described in the European patent application no.02079473.1 (PHNL021095)

In an electrowetting lens the optical power of the lens depends on thecurvature of the meniscus and the difference in refractive indicesbetween the conductive and non-conductive liquids, as can be seen in thefollowing equation:S=n ₁ −n ₂ /rWherein S is the optical power of the meniscus lens, r the radius ofcurvature of the meniscus, n₂ the refractive index of the non-conductiveliquid A and n₁ the refractive index of the conductive liquid B.

In practice there is a need to increase the range in which the power ofa variable focus lens can be varied. For example, for a zoom lens basedon electrowetting the maximum attainable zoom factor is strongly relatedto the maximum attainable change in optical power of individualelectrowetting lenses of such a zoom lens.

From the above equation follows that the optical power change of anelectrowetting lens depends on the difference in refractive indicesbetween the conducting and non-conductive liquids and on the change incurvature of the meniscus. Since the maximum change in curvature isdetermined by the size of the electrowetting cell, the change in opticalpower caused by change in curvature is limited for a givenelectrowetting lens. Moreover a strong curvature of the meniscusintroduces optical aberrations in the beam passing the electrowettinglens and requires a high control voltage. A larger optical power changecan be achieved by enlarging the difference in refractive index betweenthe conductive liquid and the non-conductive liquid. The non-conductiveliquids currently used in electrowetting lenses (e.g. alkanes orsilicone oils) have a refractive index (n=1,37-1,43) that is onlyslightly larger than the refractive index of the currently usedconductive liquids (e.g. water, n=1,33). Typically the difference inrefractive index is below 0,1.

According to the present invention at least one compound, which has atleast one aromatic, non-fused, residue, preferably at least one phenylgroup is used as the non-conducting, or non-polar, liquid or solution A,or as a component in this liquid or solution. This measure increases therefractive index in the liquid A substantially, whilst the otherrequirements for the liquid, such as high transparency, non-miscibilitywith the other liquid or fluid B and a good electrowetting behaviorstill can be satisfied.

This measure can be used to increase the range of power variations of avariable focus electrowetting lens having a given meniscus curvature orto reduce meniscus curvature of a variable focus lens having a givenrange of power variations. If used in an electrowetting zoom lens, themeasure allows increasing the zoom factor. By not-increasing ordecreasing the meniscus curvature the sensitivity for opticalaberrations in the optical system of which the electrowetting lens formsis not increased or decreased, respectively. Moreover, the requiredactuation voltage to achieve a certain change in optical power is lower.

FIG. 3 shows an electrowetting lens 20, which has the same constructionand configuration as the lens of FIG. 2, but is provided with anon-conducting fluid A′ that comprises the said compound having at leastone aromatic non-fused residue, instead of the fluid A of FIG. 2. Theresult of the replacement of fluid A by fluid A′ supplying to the lens20 a control voltage that has the same level as the voltage supplied tothe lens 1 of FIG. 2 is in the same and maintaining the level is thatthe focal spot 18′ is situated at a distance d′ from the lens, which issmaller than the distance d in FIG. 2.

For electrowetting lenses in general it is important that the meniscusshape is independent of orientation and thus of gravity. The shape willbe perfectly spherical and independent of orientation if the densitiesof the liquids are equal. This requirement can also be satisfied in theelectrowetting lens according to the invention.

A number of compounds have been traced which, if used in or as acomponent of the non-conducting fluid in an electrowetting lens, providethe required properties: high refractive index, transparent,non-miscible with the conducting fluid, a density substantially similarto that of the conductive fluid (i.e. a small difference between thedensities is allowed), proper melting and boiling points and a goodelectrowetting behavior. Examples of non-conductive liquids or solublesolids containing phenyl groups, which are very suitable to be used withthe invention are given in Table 1: TABLE 1 Refractive State DensityIndex Material Toluene Liquid 1.496 Diphenylmethane Solid 1.577 T_(m) =22° C. Biphenyl Solid 1.588 Phenyltrimethylsilane Liquid 1.49081,3,3,5-tetraphenyldimethyldisiloxane Solid 1.58661,1,5,5-tetraphenyl-1,3,3,5- Liquid 1.07 1.551 tetramethyltrisiloxane1,1,3,5,5-pentaphenyl-1,3,5- Liquid 1.093 1.5797 trimethyltrisiloxaneTriphenyltrimethylcyclotrisiloxane Liquid 1.102 1.5402 3,5,7- Liquid1.144 1.501 triphenylnonamethylpentasiloxane Reference Material:Octamethyltrisiloxane 0.82 1.38

From Table 1 it follows that the selected compounds with phenyl groupshave refractive indices typically larger than 1,49, making them suitablefor electrowetting lenses with large optical power range. Preferably,the subset with a refractive index greater than 1,5 is particularlysuited because they allow miniaturized zoom lenses for portableapplications (for instance mobile phone) with a zoom factor greater thantwo. Even more preferred are the liquids with phenyl groups withrefractive index n>1,55, for instance1,1,5,5-tetraphenyl-1,3,3,5-tetramethyltrisiloxane.

Preferably, the non-conductive liquid is a silicone oil, i.e. asiloxane, having phenyl groups. Such an oil remains long in the liquidstate on addition/substitution with more phenyl groups.

The present invention encompasses the use of phenylmethylsiloxane in atleast one of the fluid bodies of an electrowetting element, to increasethe difference in refractive index between both fluid bodies present inthe fluid chamber.

It is in this respect observed in that the not pre-publishedinternational patent application nr. IB03/00222 describes a variablefocus electrowetting lens, wherein phenylmethylsiloxane is used as acomponent of the electrically insulating liquid. However addingphenylmethylsiloxane is presented in the previous patent application asan action to give the two liquids present in the fluid chamberpreferably an equal density, so that the lens functions areindependently of orientation, i.e. without dependence of gravitationaleffects between the two liquids. The previous patent application doesnot describe the use of phenylmethylsiloxane in at least one of thefluid bodies present in the fluid chamber of an electrowetting elementto increase the difference in refractive index between both fluidbodies.

The invention may also be used in an electrowetting motor wherein use ismade of the fact that the shape of the interface can be changed by meansof an electric force, on the basis of the wetting technique, formanipulating a volume of a fluid along a predetermined path. FIGS. 4Aand 4B show a cross-sectional view of an embodiment of such a motor 30,in particular a rotary motor, at differ time moments The motor comprisesa substantially cylindrical first body 33 and a substantiallycylindrical second body 35, which is concentrically positioned withinthe first body 33. The first and second body 33, 35 enclose betweentheir respective inner and outer surface a substantially cylindricalchamber 34, which is filled with a non-polar and/or non-conductive firstfluid 36, such as an oil, and volumes 37 a-d of a polar and/orconductive second fluid 37, in this example an aqueous solution, forinstance (salted) water. The fluids 36, 37 are immiscible.

The first body 33 is provided with means for varying the wettability ofits inner surface, namely twelve electrodes 40 extending in axialdirection of the first body 33, spaced at substantially regular radialintervals along the circumference. The inner surface of the first body33 is covered with a layer 42 of electrically insulating, hydrophobicmaterial or more generally, a material having a wettability by thesecond fluid 37 which is lower than the wettability by the first fluid36. Examples of such material are for instance Teflon-like materialslike the amorphous fluoropolymer AF1600 provided by Dupont or paryleneor a combination thereof, in case where the first fluid 36 is an oil orair and the second fluid is (salted) water. The electrodes 40 areconnected to a voltage supply (not shown).

The second body 35 is of solid design but could be hollow, if sodesired, and is mounted movably, in particular rotatably, in the firstbody 33 by one or more suitable bearings. The or each bearing could forinstance be an oil bearing, configured by providing the first and/orsecond body 33, 35 with an annular groove, in which upon rotation of thesecond body 35, pressure will build up, centering the second body 35 inthe first body 33.

The second body 35 is provided at its outer surface with coupling meansin the form of four hydrophilic areas 44, said number corresponding tothe number of volumes 37 a-d. These areas 44 could for instance be madeof or covered by a material having a wettability by the second fluid 37that is higher than the wettability by the first fluid 36, whichmaterial could for instance be glass. The areas 44 are separated fromeach other in radial direction by areas 45, made of or covered byhydrophobic material, which could be a selection from any of thematerials mentioned before. Additionally or alternatively, thehydrophilic areas 44 may be recessed to enhance the coupling force withthe volumes. Furthermore, two or more of the volumes 37 a-d could beinterconnected via at least one suitable conduit 39 in second body 35,as illustrated in broken lines in FIGS. 4A and 4B. The areas of high andlow wettability 44, 45 may be omitted, but can also be maintained, toincrease the maximum force of the motor may exert.

A motor as described above operates as follows. In FIG. 4A theelectrodes 40 marked with Roman numerals I (that is the upper, lower,left and right electrodes) are supplied with a voltage. Consequently,the hydrophobic layer 42 covering said electrodes I will become locallyhydrophilic. The four volumes 37 a-d will therefore contact the firstbody 33 at the four electrodes I. They furthermore contact the secondbody 35 at the coupling means, that is the hydrophilic areas 44 and theconduits 39. If subsequently the voltage supply is shifted to secondelectrodes II, situated next tot the former electrodes I, the layerabove said second electrodes II will become hydrophilic, whereas thelayer above the first electrodes I will switch back to hydrophobic. Thisgives rise to electrowetting forces which draw the volumes 37 a-dtowards the hydrophilic areas II as shown in FIG. 4B. During thismovement the volumes 37 a-d will move along the hydrophilic area 44 ofthe second body 35 up to the edge of the hydrophobic area 45. Furthermovement along the second body 35 will be blocked by the combined actionof the hydrophobic area 45 and the first fluid 36, enabling the volumes37 a-d to exert a wetting force on the second body 35, which will causethe body 35 to rotate. Hence by sequentially activating successiveelectrodes 40 I, II with a suitable voltage, the second body 35 can berotated continuously. Preferably, the electrodes 40 are positionedrelatively close to each other or even overlap through a “tooth”structure. Also, the radial dimensions of the electrodes 40 arepreferably equal to or smaller than the radial dimensions of the volumes37 a-d. Such positioning and/or dimensioning of the electrodes 40 willensure that the volumes 37 a-d can “sense” a newly supplied voltage to asucceeding electrode 40 II.

In the given example the rotation is clockwise. It will be appreciatedthat this direction can be readily reversed by reversing the order inwhich the electrodes 10 I, II are activated. Obviously, the frequency ofrotation will depend on the activation frequency of successiveelectrodes 40 I, II. It is noted that although in the illustratedexample four volumes 37 a-d of conductive fluid are used, any number ofvolumes can be used. The volumes 37 a-d may be line-shaped in axialdirection or consist of a series of axially spaced droplets. It isfurther noted that with the embodiment of FIGS. 4A and 4B, it is alsopossible to have the first body 33 rotate instead of the second body 35,provided that the first body 33 is rotatable mounted and the second body35 is fixed. In that case, upon switching the voltage from the first Ito the second electrodes II, the volumes 37 a-d would move towards thesecond electrodes II (featuring the higher wettability) up till the edgeof the hydrophilic area 44. Subsequently, the second electrodes II dueto wetting forces would be drawn to the volumes 37 a-d, causing thefirst body 33 to rotate anti-clockwise. From this discussion it is alsoimmediately clear that for the operation of the motor 30 it isirrelevant whether the electrodes 40 are positioned on the static bodyor the movable body. Therefore, although in practice the electrodes 40will usually be placed on the static body to avoid wiring problems, thepresented embodiment should in no way be seen as limiting.

The motor described may suffer from flattening of one of the fluids dueto the exerted centrifugal force of the motor, which will influence itsperformance. According to the invention this can be prevented by usingone of the compounds described above, for example one of the compoundsof table 1. This table gives also the densities of some compounds.

The present compounds are preferably used as, or in, the non-conductingor non-polar liquid pr fluid. Because most of the compounds have adensity larger than water, which is usually the conducting liquid), itwill be obvious that said compounds should be mixed with a compoundhaving lower density, to match with the density of water.

Although the description has been limited to an electrowetting lens asan example of an optical electrowetting module, the invention is not inany way limited to such a lens. The invention may be used in any opticalelectrowetting module, such as a variable-focus lens, a zoom lens, adiaphragm, a filter and a beam deflector.

1. An electrowetting module comprising a fluid chamber, containing atleast a first body of a first fluid and a second body of a second fluidthe two bodies being separated by an interface, and means for exerting aforce on at least one of the bodies to change the position and/or shapeof the interface, characterized in that at least one of the fluidscomprises a compound having at least one aromatic, non-fused, residue.2. A module as claimed in claim 1, wherein the compound having at leastone aromatic, non-fused, residue is a compound of formula Ø-R, wherein Øis a phenyl group, substituted or not with one or more lower, C₁-C₁₀,preferably C₁-C₅ alkyl groups, and R is a linear or branched C₁-C₁₀,preferably C₁-C₅ alkyl group, being substituted or not with one or morearyl groups, an aryl group, or a silyl group, substituted or not withone or more C1-C10, preferably C₁-C₅ alkyl groups.
 3. A module asclaimed in claim 2, wherein the compound having at least one aromatic,non-fused, residue is selected from the group consisting of toluene,diphenyl methane, terphenyl and biphenyl.
 4. A module as claimed inclaim 2, wherein said compound having at least one aromatic, non-fused,residue is phenyl trimethyl silane.
 5. A module as claimed in claim 1,wherein said compound having at least one aromatic, non-fused, residueis an organosilicon oxide polymer having structural units of formula(—R₁R₂Si—O—)_(n), wherein n is an integer from 1 to 10, preferably 1 to5, R₁ is an aryl group, being substituted or not with one or moreC₁-C₁₀, preferably C₁-C₅ alkyl groups, R₂ is a lower C1-C10 alkyl group,preferably C₁-C₅ alkyl group, or an aryl group, being substituted or notwith one or more C1-C10, preferably C₁-C₅ alkyl groups, provided thatwhen n=1, and R₁ is a phenyl group, R₂ is not a methyl group.
 6. Amodule as claimed in claim 5, wherein said organosilicon oxide polymeris selected from the group consisting of1,3,3,5-tetraphenyldimethyldisiloxane,1,1,5,5-tetraphenyl-1,3,3,5-tetramethyltrisiloxane;1,1,3,5,5-pentaphenyl-1,3,5-trimethyltrisiloxane;triphenyltrimethylcyclotrisiloxane;3,5,7-triphenylnonamethylpentasiloxane.
 7. A module as claimed in claim1, wherein one of the fluid bodies comprises phenyl methyl siloxane toincrease the difference between the refractive indices of the twofluids.
 8. A module as claimed in claim 1, configured as an opticalcomponent, the first and said second fluid body having differentrefractive indices, wherein the compound added to one of the fluids hasan refractive index difference increasing effect.
 9. A module as claimedin claim 8, wherein the first fluid body is electrically conductingand/or polar, and the second fluid body is electrically non-conducting,the module being provided with means for exerting an electric force tochange the position and/or shape of the meniscus-shaped interface.
 10. Amodule as claimed in claim 8, wherein the difference in refractive indexis from 0,1 to 0,3, preferably from 0,1 to 0,2; the refractive index ofsaid second, non-conducting body being larger than 1.4, preferablygreater than 1.5.
 11. A module as claimed in claim 8, wherein said firstand said second fluid bodies show a substantially similar density.
 12. Amodule as claimed in claim 8, provided with means for exerting apressure to change the position of the interface.